1. Field of the Invention
The present invention relates to an apparatus for performing energy transfer among a plurality of battery devices interconnected in series and carried on an electric car or a hybrid car and thereby equalizing each voltage across each of the battery devices interconnected in series.
2. Description of the Related Art
The related art in this field is disclosed, for example, in Japanese Laid-Open Patent Publication No. Hei 11-176483 and U.S. Pat. No. 5,003,244. In the configuration of the former patent as shown in FIG. 7, the output voltages E1 to En of a plurality of battery devices 1-1 to 1-n are interconnected in series. For the purpose of the balance correction of the output voltages of the battery devices, a switching transistor 2 connected to a primary coil Np in series is turned ON and OFF in response to the output voltages. A converter is composed of a plurality of secondary coils Ns each corresponding to each of the battery devices and wound on a common transformer core with the primary coil. The connection thereof is configured such that the secondary output of the converter charges each battery device. When the switching transistor 2 is periodically turned ON and OFF, a voltage depending on the turn number ratio is generated in each secondary coil Ns. Since the secondary coils are wound on the common core, the induced charging current concentrates in a battery device having the lowest voltage among the battery devices, whereby the battery devices are equalized in voltage. In this former circuit, in addition to that the switching transistor is simply turned ON and OFF, the current Ip flowing in the primary coil Np is controlled depending on the load current Io.
Further, in the configuration of the latter patent as shown in FIG. 8, the output voltages of a plurality of battery devices 25, 26, 27, 28 are connected in series. For the purpose of the balance correction of the output voltages of the battery devices, a switching transistor 34 connected to a primary coil 16 in series is turned ON and OFF in response to the input from a power supply 30. A converter 14 is composed of a plurality of secondary coils 21, 22, 23, 24 each corresponding to each of the battery devices and wound on a transformer core 18 common to the primary coil 16. The connection thereof is configured such that the secondary output of the converter 14 charges each battery device. When the switching transistor 34 is periodically turned ON and OFF, a voltage depending on the turn number ratio is generated in each secondary coil. Since the secondary coils are wound on the common core, the induced charging current concentrates in a battery device having the lowest voltage among the battery devices, whereby the battery devices are equalized in voltage.
There has been the following problems in such above-mentioned related art apparatuses for equalizing the voltages across each of a plurality of energy storage device (battery devices) interconnected in series by means of the switching of a converter.
(a) In each above-mentioned related art apparatus, the magnetizing force is one directional in the transformer core for the ON and OFF duration of the switching device (transistor). Accordingly, the range of the change in magnetic flux density of the core is small, and hence the utilization of the core is less efficient. The lower efficiency in core utilization implies the necessity of a larger cross section in the core for a specific output power, thereby causing the problems of a larger apparatus and a higher cost. Further, the switching ON and OFF of the switching device for the voltage equalizing causes a problem that electric charge stored in the capacitance existing between the terminals of the switching device for the OFF duration of the switching device is discharged by the next ON operation thereby to cause a power loss and a noise due to the short-circuit current.
(b) In each above-mentioned related art apparatus, energy stored in the transformer for the ON duration of the switching device is discharged for the next OFF duration of the switching device, thereby charging a battery device having the lowest voltage among the battery devices thereby to equalize the output voltages of the battery devices. Accordingly, the amount of equalizing energy is only the amount of energy stored for the ON duration of the switching device. Therefore, in order to increase the equalizing action, a larger switching device is necessary for increasing the equalizing current. However, this larger switching device causes a larger apparatus and hence a higher cost, as is the above-mentioned case (a). Further, since each battery device has an internal resistance, the higher current from the switching device causes a larger voltage drop across the internal resistance, thereby increasing apparent output voltage of the battery device in charging. This causes a problem of reducing the precision of output voltage equalizing.
In addition to resolving the above-mentioned problems, earnestly desired are short-time equalization of the output voltages of the battery devices, reduction of energy loss after the equalization operation, and setting of the voltage at an arbitrary value after the equalization.
An object of the present invention is to resolve the above-mentioned problems (a) and (b) thereby to provide a voltage equalizing apparatus having a high efficiency and a high precision of equalizing and being of a small size. Further, an object of the present invention is to provide a voltage equalizing apparatus capable of equalizing to a desired voltage in a short time, reducing the energy loss after the completion of equalization, and setting the voltage after the equalization to be an arbitrary value.
An aspect of the invention for resolving the above-mentioned problems is a voltage equalizing apparatus for battery devices comprising:
a core;
a plurality of first battery devices interconnected in series, each consisting of one or more cells;
a plurality of secondary windings magnetically connected with each other through the core;
a plurality of a first switching devices composed of a first closed circuit by mutually connecting one of the plurality of secondary windings and the one of a plurality of first battery device;
one or more second battery devices provided separately from said plurality of first battery devices interconnected in series or a second battery device provided by rendering the whole of said plurality of first battery devices interconnected in series one battery device;
a primary winding magnetically connected with a plurality of the secondary windings through the core; and
a second switching device composed of a second closed circuit by interconnected in series between the second battery device and the primary winding;
wherein the plurality of first switching devices and the second switching device are alternately turned ON and OFF to equalize the output voltages of the plurality of first battery devices;
when the second switching device is turned ON, exciting energy stored in the core is transported by the charging to the battery devices having lower voltages among said plurality of first battery devices,
even after the transportation of said excitation energy is completed, said first switching devices continue to be turned ON, to effect the charging from the battery devices having higher voltages among said plurality of first battery devices to the battery devices having lower voltages among said plurality of first battery devices and/or to store energy in said core.